Autologous mouse testicular cells derived from the seminiferous tubules activate suppressor T cells which inhibit a syngeneic or allogeneic mixed cell reaction in vitro. Generation of cytotoxic T cells in vitro is reduced in the presence of syngeneic germ cells (spermatozoa from the seminiferous tubules or epididymal sperm). Spleen cells from mice injected with syngeneic sperm show a nonspecifically suppressed potential to generate cytotoxic T cells in vitro. In contrast, spleen cells from mice injected with syngeneic testicular cells derived from the seminiferous tubules exhibit a reduced response against TNP-modified self antigen; spleen cells from mice inoculated with allogeneic testicular cells from the seminiferous tubules suppress the cytotoxic response against splenic stimulators of the same haplotype.